1. Field of the Invention
This invention relates generally to protective relay apparatus and methods for providing single-pole tripping for ac electrical power transmission lines, and more specifically, to improved protective relay apparatus and methods for providing single-pole tripping for single-phase-to-ground faults for any value of fault resistance.
2. Description of the Prior Art
Three-phase ac electrical power transmission lines and power generating equipment must be protected against insulation faults and consequent short circuits or drops in shunt resistance that could cause collapse of the power system and serious and expensive apparatus damage. For instance, such a fault condition is caused by lightning-induced flashover from a transmission line to ground or between adjacent transmission line conductors. Under such a faulted condition, line currents can increase to several times the normal value, thereby causing loss of synchronism among generators and damaging or destroying both the transmission line and the attached equipment. To avoid equipment damage and collapse of the entire power system, faulted apparatus must be isolated from the network as fast as possible. Protective relays, which continuously monitor ac voltages and/or currents, locate line faults and initiate isolation via tripping of the appropriate circuit breakers.
Most faults on three-phase ac electrical power transmission lines are transient single-phase-to-ground faults. Conventionally, such a fault is cleared by three-pole tripping. Alternatively, the fault can be cleared by clearing only the faulted phase conductor (i.e., single-pole tripping) followed by high speed reclosing. Use of single-pole tripping enables synchronizing power to be exchanged between the other two non-faulted phase conductors and minimizes system shock, ensuring a greater degree of stability. A large generation plant, for example, connected to the electrical power system via a single transmission line, retains synchronization when single-pole tripping is used. Three-pole tripping requires the power generating machines to be resynchronized before reconnection to the system.
In single-pole tripping, the protective relay must distinguish a single-phase-to-ground fault from a phase-to-phase fault, a double-phase-to-ground fault, and three-phase faults, and must correctly identify the faulted phase conductor. Several techniques are available for providing single-pole tripping, including: evaluation of the zero or negative sequence currents in a pilot relaying scheme; traveling-wave evaluation (see U.S. patent application Ser. No. 404,170, filed Aug. 2, 1982, now U.S. Pat. No. 4,438,475, and assigned to the assignee of the present invention), and segregated phase comparison.
Additionally, U.S. patent application Ser. No. 321,158 (filed Nov. 13, 1981), now U.S. Pat. No. 4,398,232, discloses a novel apparatus and method for detecting a single-phase-to-ground fault on a three-phase electrical power system, and for identifying the faulted phase conductor. In this prior art technique, each phase conductor is monitored using phasor signals representative of: (1) the sum of the three phase currents, (2) the negative sequence current or voltage of the monitored phase conductor, and (3) the phase-to-phase voltage that is in quadrature with the phase-to-ground voltage of the monitored phase. A predetermined phase angle relationship between these three phasor signals for a phase conductor identifies that conductor as the one experiencing a fault to ground. The predetermined phase angle relationship is these three phasor signals within 60 electrical degrees of each other. For a 60.degree. relationship, the time of coincidence is at least 5.55 ms for a 60 Hz system. It has been determined that this prior art approach using three phasor signals cannot properly detect close-in faults having a high resistance to ground. Thus, the instant invention improves upon the prior art technique by disclosing means to protect against high resistance close-in faults.